This invention relates to mass spectrometry and ion mobility spectrometry, and more particularly is concerned with a hybrid mobility-mass spectrometry apparatus and a new method of using such a hybrid device.
Presently, there are a wide variety of different analysis techniques known for analyzing solvents and substances of interest.
Fundamentally, all mass analysis instruments operate at low pressures, at least in the mass analysis section. As such, separation of different ions depends solely upon different mass-to-charge ratios of the ions present. A problem thus arises where one has two similar ions which happen to have an identical or similar mass-to-charge ratio. Such ions are considered to be isobaric, and cannot be separated by conventional mass spectrometry techniques.
Another known technique for analyzing substances is ion mobility spectrometry (IMS). In such a system, a substance to be analyzed is ionized, to the extent possible, as is required of the low pressure mass spectrometry technique detailed above; however, the techniques for ionization unnecessarily differ due to the different pressures and operating conditions. IMS is commonly carried out at higher pressures, even at atmospheric pressure, and can even use ambient air. However, it is often preferred to use some known, selected gas which is dry, clean and pure and has known properties. Ions are then caused to travel down a drift tube under a potential gradient, through the gas. Different ions have different mobility characteristics depending upon the size and type of the ion and its charge. Thus, different ions will have different transit times to traverse the drift tube. Ions are detected at a detector at an exit from the drift tube, and, knowing transit times for different ions, the constituent components of a sample can be determined.
A drawback with IMS is that it can provide only poor resolution (approximately 100 for example) as compared to other known mass spectrometers. The problem is related to diffusion of the gas in the drift tube. In contrast, the low pressure mass spectrometry techniques detailed above can provide high resolution (for example, approximately 10000) and consequently can distinguish between ions having close but different mass-to-charge ratios.
Again, there is a problem with IMS techniques in that one can encounter substances that have similar drift times but are in fact quite different. Such substances cannot be resolved or separated by IMS.
There are also other known separation techniques relying on quite different technologies, such as chromatography and electrophoresis. For example, liquid chromatography involves passing a sample in a liquid phase through a chromatography column. The column is provided with a packing, selected to provide different retention properties for substances of interest. Then, by analyzing substances as they leave the chromatography column and measuring the time taken to traverse the column, an initial sample can be broken down into its separate portions.
Another known separation technique, electrophoresis, in turn relies upon the fact that different ions will have different mobilities in a liquid phase. A DC voltage or potential gradient is applied to a column, typically made of a liquid or gel, and a starting substance or sample to be analyzed is injected at the entrance end of the electrophoresis column. The potential gradient causes different components of the sample to traverse through the gel at different rates, due to their different mobilities. Again, this enables different components to be detected as they leave the electrophoresis column. Alternatively, at some point the potential gradient can be turned off, so as to fix the different components at different physical locations within the gel, which then can be physically broken into separate portions for analysis.
Accordingly, it has been recognized by a number of workers in this field that there is some advantage in providing so called two-dimensional separation techniques. In liquid chromatography and electrophoresis, there have been proposals which involve taking a sample, subjecting it to a first separation technique and then another separation technique of the same type. For example, in electrophoresis, a sample can be subjected to electrophoresis separation in a gel of one type, and then a second electrophoresis separation step in a second gel having different characteristics, intended to separate out any constituents present which may have had identical characteristics in the first gel.
Such separation techniques are often considered to be xe2x80x9corthogonalxe2x80x9d, since the two separation steps are wholly independent of one another. Moreover, the results can be presented as a two-dimensional chart, with orthogonal axes, where each axis represents one of the separation steps.
Moreover, there has been a proposal for combining quite different separation techniques. For instance, there has been a proposal to combine liquid chromatography or electrophoresis with some type of mass spectrometry. This can present a number of difficulties.
Firstly, a sample from liquid chromatography or electrophoresis has to be processed so as to be in a form suitable for generation of ions from mass spectrometry. For example, many modern mass spectrometers use an electrospray technique. The sample thus has to be introduced to an electrospray source, while maintaining any resolution obtained from the previous electrophoresis separation technique or the like. Earlier PCT patent application No. PCT/CA99/00868 demonstrates one proposal for such a technique.
Another fundamental problem is that the sample in capillary electrophoresis or liquid chromatography is carried out in a buffer. Once the sample is electrosprayed the mass spectrum will feature peaks related to the sample and also a wide range of peaks related to the buffer. These buffer related peaks are commonly called xe2x80x9cchemical noisexe2x80x9d. It is the chemical noise that often imposes limits on the detection of the minute amounts of sample. Additionally, techniques such as electrophoresis are labor intensive as a gel has to be prepared for each run.
Low pressure mass spectrometry, which inherently depends solely on the mass-to-charge characteristics of each ion, and ion mobility spectrometry (IMS) have been considered to be two different but similar techniques. They are considerably different, since they inherently rely on different techniques to achieve separation. At the same time, there are significant similarities; IMS relies on different mobilities of ions in a gas phase; low pressure mass spectrometry while, ideally, taking place in an absolute vacuum, necessarily has some gas pressure present, and additional steps, such as collisional fragmentation, inherently require the presence of a significant gas pressure thereby providing some, superficial similarity with IMS.
U.S. Pat. No. 5,905,258 (Clemmer) discloses a Hybrid ion mobility and mass spectrometer and there have been other proposals for a hybrid spectrometer (Fuhrer et al. Anal. Chem. 2000, 72, 3965-3971). These proposals recognize that there are significant advantages in combining an IMS technique with a low pressure mass spectrometry technique. Such hybrid instruments provide the advantages of two different separation techniques, thereby enabling separation of two or more constituents or ions which, in either one of the techniques, have similar characteristics preventing separation.
The ion mobility step can be operated at a pressure much less than atmospheric pressure, so as to enable it to be fairly readily combined with a low pressure MS technique, without imposing any undue requirements with respect to pumping or maintaining separation between different chambers and the like. The main problem of a low pressure mobility separation setup is in the resultant high rate of diffusion. Losses of the ions occur when the diameter of the ion beam becomes bigger than the diameter acceptable for mass spectrometer. It has been proposed to use a multipole ion guide with an axial field to overcome the diffusion problem in U.S. Pat. No. 5,847,386. The multipole ion guide can confine the ion beam and even reduce the beam diameter so that it will become acceptable for mass analysis.
One aspect of the present invention is to provide an ion mobility spectrometer having a rod set to promote confinement of the ions to the axis. The DC draft field or gradient can be provided in many ways. It is preferably provided by forming the rod set as a segmented rod structure. The individual segments of each rod can then be provided with a differing DC potential to establish the potential gradient.
In accordance with another aspect of the present invention, there is provided a spectrometer comprising: an ion mobility spectrometer (IMS) device, for use in promoting separation of ions based on different mobility characteristics, the ion mobility spectrometer device comprising: an inlet for ions; a rod set having an axis and comprising a plurality of individual rods arranged around an axis; means for applying an RF voltage to the rod segments for focusing ions along the axis; and means for forming a DC field within the rod set, to generate a potential gradient along the device;
and means for maintaining a gas pressure within the rod set whereby ions travelling through the rod set under the influence of potential gradient are subject to collision with the gas, promoting separation based on differing mobility characteristics; and
at least one mass analysis section, providing a first mass analysis section, for receiving ions from the ion mobility spectrometer device and for separating ions based on differing mass-to-charge characteristics.
As mentioned, segmented rods can be provided. Alternatively, the means for forming the DC field comprises one of: auxiliary elements located around the rod set and connected to a power supply for generating the DC field and the potential gradient; and providing the rods of the rod set with inclined surfaces whereby a potential gradient can be formed.
The IMS section can include an upstream ring guide section where the pressure is relatively high, as focusing of ions with a rod set is poor at high pressures.
The mass analysis section can comprise a time-of-flight mass analyzer or a quadrupole rod set with a detector, for example. Additionally, for MS/MS analysis, a collision cell and a second mass analyzer can be provided.
Another aspect of the present invention provides a method of separating ions based on ion mobility characteristics, the method comprising:
(i) generating ions;
(ii) providing a drift region having an axis extending therealong and providing a rod set having a plurality of rod segments, with the drift region being located within the rod set, and maintaining a gas at a desired pressure in the drift region;
(iii) applying an RF voltage to the rod set to maintain desired ions focused along the axis of the rod set; forming a DC potential gradient along the rod segments of the rod set;
(iv) supplying ions to the drift region, whereby ions are driven through the drift region by the potential gradient and ions tend to separate due to differing ion mobility characteristics, and
(v) passing ions into a mass analyzer for mass analysis in dependence upon ion mass-to-charge ratios.
Preferably, the method includes separating ions into groups of ions in step (iv) in dependence upon ion mobility characteristics, and sequentially analyzing each group of ions in step (v).
More preferably, the method includes establishing for each group of ions an approximate range of mass-to-charge ratios present in the group, and mass analyzing the ions in step (v) in a Time-of-Flight mass analyzer, and setting timing of the Time-of-Flight mass analyzer in dependence upon the range of mass-to-charge ratios present in each group, thereby to enhance the sensitivity of mass analysis in the Time-of-Flight mass analyzer.
Another aspect of the method includes, between steps (iv) and (v), passing the ions through a collision cell to promote formation of product ions; by one of fragmentation and reaction, and subsequently mass analyzing the product ions in step (v).
More preferably, before passing the ions into the collision cell, the method includes subjecting the ions to an upstream mass analysis step, to select a desired precursor ion for said at least one of fragmentation and reaction, and periodically resetting the precursor ion selected in said upstream mass analysis step, as different ions pass out from step (iv), thereby to increase utilize usage of ions from a sample.
The present invention also provides for utilization of a segmented rod structure to form wells for trapping ions, after separating ions based on their mobility characteristics. Then the ions in each well can be separately released for mass analysis, collision and subsequent mass analysis, or any other purpose.
A further apparatus aspect of the present invention provides an apparatus comprising: an ion mobility spectrometer device, for use in promoting separation of ions based on different mobility characteristics, the ion mobility spectrometer device comprising an inlet for ions; a drift region; means for forming a DC field along the drift region, to generate a potential gradient along the drift region; and means for maintaining a gas pressure within the drift region, whereby ions travelling through the drift region under the influence of a potential gradient are subject to collision with a gas, promoting separation based on differing ion mobility characteristics;
a collision cell connected to the ion mobility spectrometer for receiving ions therefrom and including a gas therein, for promoting at least one of fragmentation of ions and reaction of the ions with ambient gas, to form product ions; and
a final mass analysis section for analyzing the product ions.
The method aspect of the present invention also provides a method for separating ions based on ion mobility characteristics, the method comprising:
(i) generating ions;
(ii) providing a drift region having an axis extending there along;
(iii) forming a DC gradient along the drift region;
(iv) supplying ions to the drift region, whereby ions are driven through the drift region by the potential gradient, thereby to promote ion separation due to differing ion mobility characteristics;
(v) passing the ions into a collision cell to promote at least one of fragmentation and reaction with a collision gas, thereby to generate product ions;
(vi) subjecting the product ions to mass analysis.
Yet another aspect of the method of the present invention provides a method of operating a spectrometer system comprising an ion mobility section connected to a time-of-flight mass spectrometer, the method comprising;
(i) generating ions;
(ii) supplying the ions to the drift region of the IMS section;
(iii) forming a DC potential gradient along the drift region of the IMS section, thereby to promote separation of ions based on differing mobility characteristics;
(iv) supplying ions eluting from the ion mobility section to the time-of-flight mass spectrometer; and
(v) adjusting the duty cycle of the time-of-flight mass spectrometer to correspond to the range of mass-to-charge ratios in each group of ions received from the ion mobility section, thereby to enhance the overall duty cycle of the time-of-flight spectrometer.